The control of the cell cycle in microbial symbionts



The development of the photosynthetic microbial symbionts of lichens and Azolla are compared. Both have more rapidly dividing cells at the growing point and slower rates of cell division in the mature regions of the host symbiont. Cell size, numbers and differentiation (function) were studied in these different stages of development. Anabaena cell number in the mature leaf cavities of Azolla increases more or less linearly. When fern growth was stopped with cycloheximide treatment, cell division in Anabaena at the apex was also arrested. In lichens, photobiont cell size increases with thallus age, us does Anabaena in Azolla. Lichen photobiont cells became larger in slower growing thalli. The pattern of photosynthetic carbon and nitrogenase functions change during thallus development. These observations together with others are used to suggest two separately operating controls on the symbiotic cell cycle: (1) nutrient supply for cell maintenance and cell growth; (2) constraint or stimulation of commitment to divide. The separate operation of these controls leads to symbiont cells being in two possible states: (1) phase I which are generative cells associated with growing host tissues and (2) phase II which are differentiated cells in the mature regions of the host.